The world is divided longitudinally into time zones, with each hour difference roughly 15 degrees apart (one earth rotation, 360 degrees/ 24 hours). Due to some countries have half hour time zones, there are more than 24 times zones in the world. The starting point at which Coordinated Universal Time (UTC) starts is the Prime Meridian (zero degrees longitude) which we call as Greenwich Meridian Time (GMT). This passes roughly through London. Therefore, -7.5 degrees (West) to +7.5 degrees (East) is considered to be GMT time zone.

To calculate time difference between the two different time zones, you must start by finding the difference in longitude (or degrees) of the respective two places, in respect to the GMT.

For straight and simple calculation, just check your time zone from Google and use the Time Zone Convertor